The intermediate processing of stock is known in the art as converting. The basic function of the converting industry is to convert flexible materials, such as paper, plastic, cardboard, light foil and the like, for use in the packaging industry. The conversion of these materials includes printing, slitting, rewinding, trimming, etc. A large volume of the material converted is in roll form.
In the various stages of the converting and handling of packaging material in roll form, many situations, conditions, and mistakes occur that causes material to be discarded or wasted. Many of the problems that arise are inherent in the machines that process and rewind the material. Much of the discarded material is capable of being salvaged.
There is a need for machines to reprocess and salvage this valuable material. Such machines must be capable of producing reprocessed rolls of material which are reconstructed and finished with the same quality and conformation as the original order specification requires.
Because of the variable characteristics of the packaging materials, one machine cannot handle all types of materials. This is especially true for the thin, stretchably deformable plastic webs. Such plastic webs take a variety of forms such as sheeting, tubing, multi-ply film, etc.
Often there is a need for reprocessing pre-printed plastic web on which printed matter appears in a predefined regular interval. In reprocessing such material, it is imperative that the plastic film is not stretched which would adversely effect the registration of the printed matter over the length of the plastic web.
Preventing stretch is particularly problematic in conjunction with rewinding the reprocessed material to produce an acceptable rewound roll. There are two types of rewinders in general use for rewinding plastic pre-formed web material onto storage rolls, namely surface driven rewinders and center driven rewinders.
Surface driven rewinders include one or more driven rollers whose surface makes contact with the outside layer of material being rewound on a freely rotatable core. The surface rewinder's driven rollers are driven at a speed slightly higher than that of the moving web so as to create a positive pull or tension.
A major disadvantage of this type of machine is that the pressure on the driven rollers increases as the roll diameter increases due to the ever increasing weight of the rewind roll as it is being rewound. As a result, a minor amount of air is squeezed out of the inner layers and a major amount is squeezed out of the outer layers. This makes for a soft inner roll and a hard outer roll; this can cause problems when the roll is placed in a different machine for further processing. This can also cause telescoping and irregularly shaped rolls. In order to maintain equal surface pressure throughout the build-up of the roll, it is necessary to provide additional mechanical, pneumatic or hydraulic counterforce mechanisms. These mechanisms add to the complexity and cost of the machine.
In center driven rewinders, the spindle on which the web of material is rewound to produce the rewind roll is driven. The web is pulled by the rewind roll as succeeding layers of material are continuously applied.
It is problematic to maintain constant tension on the web to achieve uniform web pull, since the weight of the rewind roll increases and the circumferential speed of the growing roll also increases. The increasing weight requires an ever increasing motor torque in proportion to roll diameter. Further, the speed of a center driven core is in inverse proportion to roll diameter. Although the driven core's speed must be reduced as the rewind roll's circumference increases, the required horsepower must remain constant throughout the roll build-up.
Few, if any, constant horsepower drives have heretofore been able to provide a constant horsepower output throughout the build-up range of a rewind roll. Further, center driven rewinders usually employ excessive tension on the web to remove the air between layers of material. This can result in stretching and eventual tearing of the web, particularly when the web is a thin plastic material.
Freely rotating doctor rolls are known in the art which engage the surface of a center driven rewind roll to facilitate the removal of air from the roll. However, the center driven rewinder is still required to exert a pulling force on the web to maintain web tension.